Modern functional materials, for example ferromagnets and piezoelectrics, are typically chemically complex and exhibit the co-existence of multiple phases that evolve as a consequence of chemical alloying. In such materials, huge responses to external stimuli are often found at phase boundaries. In the past two decades, examples of the discovery of such behavior include the emergence of colossal magnetoresistance in doped manganites, high temperature superconductivity in doped cuprates, and large piezoelectric responses in relaxor ferroelectrics. The large piezoelectric coefficients in Pb(Zrx,Ti1-x)O3 (PZT), Pb(Mg0.33,Nb0.67)O3—PbTiO3 (PMN-PT), and Pb(Zn0.33,Nb0.67)O3—PbTiO3 (PZN-PT) systems, for example, occur in compositions that lie at the boundary between two crystal structures, e.g., a rhombohedral-to-tetragonal phase boundary. These giant piezoelectric responses have made PZT, PMN-PT, and PZN-PT the materials of choice for a variety of applications ranging from micro-positioners to acoustic sensing in sonar. Notwithstanding the dramatic progress in the development of functional piezoelectric devices from these lead-based perovskites, two broad challenges remain: (i) lead-free alternatives to the above-mentioned systems, and (ii) viable alternative pathways that are fundamentally different from the chemical alloying approaches (such as that seen in the PZT and PMN-PT systems) to achieve large piezoelectric responses.